1. Field of the Invention
The present invention relates to a TM (transverse magnetic) mode dielectric resonator in which a dielectric pillar is disposed in a space surrounded by a conductor.
2. Description of the Related Art
FIG. 14 and FIG. 15 show the structure of a conventional TM mode dielectric resonator. In FIG. 14, numeral 1 designates a prismatic dielectric pillar in which a frequency adjusting hole 2 is formed in a direction orthogonal to the axial direction of the dielectric pillar. Numeral 3 designates a cavity which is integrally formed with the dielectric pillar 1. A conductor 4 is formed on the top and bottom faces and on the left and right side faces of the cavity 3. A hole 11 is formed in the cavity 3 for guiding a frequency adjusting dielectric rod for movement into and out of the frequency adjusting hole 2 in the dielectric pillar 1. Two opening faces of the cavity 3 are covered with metallic panels 5 and 6.
FIG. 15 is a view showing a central vertical section of the TM mode dielectric resonator shown in FIG. 14, including a frequency adjusting dielectric rod. In FIG. 15, numeral 7 designates a frequency adjusting dielectric rod, numeral 8 designates a screw member for holding the frequency adjusting dielectric rod 7 and numeral 9 designates a holding member attached to the hole 11 formed in a side wall of the cavity 3. The screw member 8 is threaded to engage with the holding member 9.
FIGS. 16(A) and 16(B) show examples of electric fields created in the dielectric pillar of the TM mode dielectric resonator shown in FIG. 14 and FIG. 15. As shown in FIG. 16(A), in a region of the frequency adjusting hole 2 into which the dielectric rod is not inserted, electric lines of force pass almost totally through the dielectric portion of the dielectric pillar, detouring the frequency adjusting hole 2. By contrast, as shown in FIG. 16(B), in a region of the frequency adjusting hole 2 into which the dielectric rod 7 is inserted, the electric lines of force concentrate on the dielectric rod 7 in the frequency adjusting hole 2. In this way, an effective dielectric constant of the entire dielectric pillar is changed by inserting and withdrawing the frequency adjusting dielectric rod, whereby the resonance frequency is changed.
FIG. 17 and FIGS. 18(A) and 18(B) show an example of a TM mode dielectric resonator using a double mode, having a composite dielectric column having the shape of two intersecting dielectric pillars. As shown in FIG. 17, a composite dielectric pillar 1 has a shape of two intersecting dielectric pillars in which frequency adjusting holes denoted by 2x and 2y are installed. Holes 11x and 11y are provided in a side wall of the cavity 3 for holding frequency adjusting dielectric rods in the frequency adjusting holes 2x and 2y, respectively, so that they can be inserted and withdrawn. A hole 12 is also formed in a side wall of the cavity 3, for holding a coupling adjusting member which adjusts the coupling coefficient between two resonators formed by the two dielectric pillars constituting the composite dielectric pillar 1.
FIGS. 18(A) and 18(B) are a top view and a sectional view of the resonator shown in FIG. 17, respectively. In FIGS. 18(A) and 18(B), screw members 8x and 8y are threaded to engage with holding members 9x and 9y, respectively, and frequency adjusting dielectric rods 7x and 7y, respectively attached to end portions of the screw members 8x and 8y, are respectively inserted into and withdrawn from the dielectric pillars by turning them, thereby adjusting the frequency of the resonator comprising the dielectric pillars extending in the horizontal and the vertical directions. Further, in FIGS. 18(A) and 18(B), numeral 13 designates a dielectric rod for coupling adjustment between the two pillars that is threaded to engage with a holding member 14. The coupling coefficient between the two resonators comprising the two dielectric pillars is adjusted by inserting and withdrawing the coupling adjusting dielectric rod into and from one of four corner portions produced by the intersection of the two dielectric pillars.
However, as discussed further below, the available frequency adjusting range is narrowed by downsizing a TM mode dielectric resonator having a structure in which cylindrical frequency adjusting dielectric rods are inserted into and withdrawn from frequency adjusting holes having a circular sectional shape, as is illustrated in FIGS. 14, 15, 16(A), 16(B), 17, 18(A) and 18(B). Further, a coupling adjusting range is narrowed by downsizing a TM mode dielectric resonator having a structure in which a coupling adjustment is performed by inserting and withdrawing a coupling adjusting dielectric rod into and from a space inside a side wall of a cavity as is illustrated in FIGS. 17, 18(A) and 18(B).
That is, the dimensions of the dielectric pillars are determined in compliance with a frequency of use, and accordingly, in downsizing the overall TM mode dielectric resonator, the outer dimensions of the cavity are reduced as a result, whereby a ratio of the volume of the dielectric pillars to that of the cavity is increased and a distance between the dielectric pillar and the inner wall of the cavity denoted by S in FIG. 15, is shortened. As a result, the frequency adjusting dielectric rod 7 has an insufficient movable range (stroke). The stroke of the frequency adjusting dielectric rod must be further restricted to prevent interference with, for example, an input and output coupling loop, etc., due to the narrowing of the spaces between the metallic panels covering the opening portions of the cavity and the dielectric pillars, and between the inner walls of the cavity and the dielectric pillars. Finally, the range over which the frequency is variable is considerably restricted by downsizing the TM mode dielectric resonator. Further, with respect to a double mode resonator using a composite dielectric pillar, the adjustable range of the coupling coefficient is restricted since the coupling adjusting dielectric rod has an insufficient movable range.
To enlarge the frequency adjusting range given the limited movable range of the frequency adjusting dielectric rod, a ratio of frequency change relative to a moving distance of the frequency adjusting dielectric rod must be enhanced. It is effective for that purpose to enhance, for example, the dielectric constant of the frequency adjusting dielectric rod or to enlarge its sectional area. However, the dielectric constant is determined inherently by the materials that are usable for the frequency adjusting dielectric rod, and accordingly, the dielectric constant cannot considerably be enhanced. Further, spaces between the inner walls of the cavity or the metallic panels and the dielectric pillars are limited, and therefore, even if the frequency adjusting dielectric rod is enlarged, the structure of the holding member and the like for holding the rod also must be enlarged. Therefore, there is a limit on how much the frequency adjusting dielectric rod can be enlarged. The same limitations are also applicable to enlarging the coupling adjusting dielectric rod.